High-quality imaging requires a large dynamic range to render all possible grey levels. While cameras have made great progress both in terms of dynamic range and resolution, displays for playing back such images are still missing, except for high-end medical displays. With advanced cameras entering mobile phones, there is an increased need to playback images accurately also on consumer displays, particularly mobile displays. In order to render the entire dynamic range of the image, the display needs to 1) exhibit large contrast ratio both in dark and bright environments and 2) have a large number of addressable grey levels. To maintain readability in bright environments, it is common to tune the tone rendering curve (TRC) or gamma according to ambient light. In order to do that, however, more grey levels are needed in order not to lose the number of distinguishable grey levels. Mobile displays have traditionally been utilizing transflective displays to preserve the contrast and thus readability in a variety of luminous environments. However, there is a trade-off between color reproduction range (gamut) and luminance in the reflective mode so some recent displays have abolished reflective color all together in favor of higher monochrome contrast and brightness, thus enhancing readability in bright environments. Problems with transflective displays are high manufacturing costs and limited resolution compared to transmissive or fully reflective displays. This limited resolution and the need for wider gamut has led to a trend towards more transmissive and emissive displays. In order to achieve outdoor readability, however, a large luminance and hence a high-power backlight, is needed. For conventional transmissive or emissive displays, currently there is no method for trading color range for brightness in the same way as for transflective displays—trading is fixed and determined by the reflective/transmissive aperture ratios and the color filter spectra. By contrast, a recently proposed field-sequential-color display with adaptive gamut can provide a continuous exploitation of this trade-off. However, it is limited to the same (small) number of addressable grey levels as conventional displays and therefore cannot render images with a large dynamic range.
Electronic displays based on an analogue material response, e.g., liquid crystal displays (LCDs) or organic light emitting diode displays (OLEDs) require a digital-to-analog converter (DAC) in order to translate the digital image data to actual images on the display. Because of cost and power considerations, the resolution of these DACs is limited, particularly for mobile displays. The widely used 6-bit DACs enable a 6 bit grey scale for each primary which, for an RGB display gives 18 bits per pixel (bpp) or 23×6=262,144 addressable colors. This color depth is often extended to 24 bpp by using frame rate control (FRC), i.e., temporal averaging of several frames to achieve the extra 6 bits needed. While this is a cost and power efficient solution, it sacrifices moving image quality since the refresh rate for certain grey levels is lowered.
Increasing the resolution of DACs is straightforward but results in higher cost and increased power consumption, even for moderate color depths like 24 bpp. 10-12 bit DACs are available for medical displays but they are expensive and need precise calibration with laser-tuning. The FRC provides a simple means of extending the color depth but at the expense of moving image quality. Since conventional displays have fixed primary colors, there is no way to trade color for dynamic range. Thus the dynamic range of the primary color with least bpp will limit the dynamic range in monochrome or multi-color images.
Conventional LCDs and OLEDs are spatially divided into picture elements (pixels) which, in turn, are spatially divided into individually addressable subpixels which represent each primary color, e.g., RGB (red, green, blue). In the case of LCDs, white light from the surroundings (reflective displays) or from the backlight (transmissive displays) is filtered through primary color filters on the subpixels to form pixels of any color. Field sequential color displays (FSCDs) are transmissive displays without subpixels or color filters and the image is instead formed by a sequence of images separated into each primary color, e.g. RGB. This sequence is faster than the integration time of the human visual system (HVS) so the colors are “fused” in the brain